Embodiments of the invention relate generally to electric drive systems including hybrid and electric vehicles and to stationary drives that are subject to transient or pulsed loads and, more particularly, to transferring energy between an energy storage device of the vehicle or drive and a power source external to the vehicle or drive to rapidly charge the energy storage device.
Hybrid electric vehicles may combine an internal combustion engine and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration.
Plug-in electric vehicles, whether of the hybrid electric type or of the purely electric type, are configured to use electrical energy from an external source to recharge the traction battery. Such vehicles may include on-road and off-road vehicles, golf cars, neighborhood electric vehicles, forklifts, and utility trucks as examples. These vehicles may use either off-board stationary battery chargers or on-board battery chargers to transfer electrical energy from a utility grid or renewable energy source to the vehicle's on-board traction battery. Plug-in vehicles may include circuitry and connections to facilitate the recharging of the fraction battery from the utility grid or other external source, for example. The battery charging circuitry, however, may include dedicated components such as boost converters, high-frequency filters, choppers, inductors, and other electrical components dedicated only to transferring energy between the on-board electrical storage device and the external source. These additional dedicated components add extra cost and weight to the vehicle.
In addition, on-board or off-board rectifiers may be used to provide high charging currents for rapidly charging energy storage devices. However, such rectification has the undesirable effects of high utility harmonics and a low power factor due to phase shift of the utility fundamental waveforms. The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power and may be expressed as a number between 0 and 1 or as a percentage between 0 and 100. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power. A circuit with a lower power factor performs less work than a circuit with a higher power factor. Therefore, to perform the same amount of work, a higher voltage or current is input into the circuit with the lower power factor.
While a power factor correction capacitor may be used to improve the low power factor resulting from use of the rectifier, the size of such a correction capacitor is large due to the low frequency of the utility (e.g., 50 or 60 Hz). Further, a power factor correction capacitor has minimal effect on lower-frequency harmonic currents that also contribute to the low power factor and can lead to unexpected system resonances. Dedicated active filters may also be provided, however, implementing such filters adds additional costs to the system in the form of a full-power-rated power electronic converter, which processes the complete charging power using expensive switching devices, and associated controls.
It would therefore be desirable to provide an apparatus to facilitate the rapid transfer of electrical energy from an external source to the on-board energy storage device of a plug-in vehicle that reduces the number of components dedicated only to transferring energy between the on-board electrical storage device and the external source and reduces the charging system costs, while maintaining a high power factor and low harmonic currents.